How to When/thenReturn a method of a non-attribute of a class

Question

I have a method in a class as follows:

class Foo{

    public void foo(){
       Bar bar = new Bar();

       String barResult = bar.something();
       //code to test
    }

}

Note that bar is not an attribute of Foo class. I need to write a test case for foo() and I need a way to mock bar.something().

E.g: when(bar.something()).thenReturn('myresult');

But it is not possible since bar is not accessible from the test class. How can I achieve this?